Hitherto, a various technologies for protecting an occupant by providing an airbag apparatus on a motorcycle are known. For example, in a motorcycle, a technology in which an airbag stored in a case mounted to a vehicle body frame is deployed and expanded by expansion gas in case of a front collision, whereby restraining the occupant, is publicly known, for example, see Japanese Unexamined Patent Application Publication No. 2002-137777. In this technology, although a possibility of securing a wide protective area for the airbag is presented, in a case of mounting the airbag apparatus on the vehicle body which is opened on all four sides as a motorbike, development of a technology which is further effective for assuring restraint of the occupant by the airbag is highly demanded.